Add contrast colors Sass map

[jacobmllr95]

This commit adds a $contrast-colors Sass map to the variables. This makes it possible to define a custom contrast color for a given color. Closes #23418.

Let's say someone wants the color for btn-secondary to be black:

$contrast-colors: (
  $gray-600: $black
);

Or if you want to restore the Bootstrap v4 Alpha colors:

$blue:   #0275d8;
$green:  #5cb85c;
$teal:   #5bc0de;
$yellow: #ffd500;
$red:    #d9534f;

$theme-colors: (
  secondary: $white
);

$contrast-colors: (
  $blue: $white,
  $white: $gray-600,
  $green: $white,
  $cyan: $white,
  $yellow: $white,
  $red: $white
);

[mdo]

Closing for a simpler approach of adding vars for these two colors: #24426. It's not the per-theme-color approach as here, but it provides some flexibility.